1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image onto a recording medium under application of heat and pressure, to an image forming apparatus including the fixing device, and to a method of connecting wires in the fixing device.
2. Description of the Related Art
There has been widely used a fixing device of heat-roller fixing type as a fixing device for use in an electrophotographic image forming apparatus such as a copying machine and a printer. The fixing device of heat-roller fixing type includes a pair of rollers (a fixing roller and a pressure roller) that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen lamp, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined temperature (a fixing temperature). With the pair of rollers kept in a heated state, such as a recording paper sheet, which is a recording medium having formed thereon an unfixed toner image, is fed to a region where the pair of rollers make pressure-contact with each other (a fixing nip region). Upon the recording paper sheet passing through the pressure-contact region, the toner image is fixed to the recording paper sheet under application of heat and pressure.
Incidentally, a fixing device for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixing on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
In the fixing device provided in such a color image forming apparatus, for high speed, it is necessary to widen the nip width of the fixing nip region. As a method for widening the nip width, there is a method in which the thickness of the elastic layer of the fixing roller increases, or a method in which the diameter of the fixing roller increases. However, in the fixing roller having the elastic layer, the thermal conductivity of the elastic layer is very low. For this reason, when a heating section is provided in the fixing roller, if the process speed increases, the temperature of the fixing roller may not follow the process speed. Meanwhile, when the diameter of the fixing roller increases, the warm-up time may be extended or power consumption may be increased.
As a fixing device provided in a color image forming apparatus to solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device of belt fixing type that is configured so that a fixing belt is supported around a fixing roller and a heating roller, and the fixing roller and a pressure roller are brought into pressure-contact with each other with the fixing belt interposed therebetween. In the fixing device of belt fixing type, since the fixing belt with a small heat capacity is heated, it takes short time to warm up and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness made of sponge rubber and the like and to secure a wide nip width.
Furthermore, JP-A 2002-333788 discloses, as a fixing device of belt fixing type, a fixing device of planar heat generating belt fixing type with a heating section as a planar heat generating element (i.e., a heat generating element which is configured such that a heat generating resistor forms a predetermined shaped plane as a whole). In the fixing device of planar heat generating belt fixing type, since a heat capacity of the heating section is reduced and the planar heat generating element as the heating section directly generates heat, a thermal response speed is also enhanced compared to a system in which a heating roller is heated indirectly using a halogen lamp or the like and it is possible to attain further shortening of a time for warm up and more energy saving.
The fixing device of belt fixing type also includes a temperature detection element, such as a thermistor, which detects the surface temperature of the fixing belt, and an overheat preventing element (a thermostat, a temperature fuse, a thermal protector, or the like) which detects an abnormal temperature rise of the planar heat generating element being in an overheat state. In such a fixing device, electrical conduction to the planar heat generating element is controlled on the basis of temperature data detected by the temperature detection element and abnormal temperature rising data detected by the overheat preventing element such that the surface temperature of the fixing belt is at a predetermined temperature (fixing temperature).
In the fixing device of belt fixing type using a high-power-density planar heat generating element, when the temperature detection element is not operated normally due to the wires of the temperature detection element being loosened, and electrical conduction to the planar heat generating element is not controlled normally, abnormal electrical conduction is provided to the planar heat generating element. The high-power-density planar heat generating element has a high temperature rising speed. Thus, if abnormal electrical conduction is provided to the planar heat generating element even in a short time, the planar heat generating element may be in the overheat state and may result in smoke generation or burnout, or may result in smoke generation or ignition due to heat generation caused by electrical conduction in an incomplete connection state and a half-short-circuit state. Further, if abnormal electrical conduction to the planar heat generating element in a short time is repeated, the overheat state of the planar heat generating element is continued, the fixing belt may be damaged and then deteriorated, or may result in smoke generation or ignition.